1. Field of the Invention
The present invention relates generally to sensing and control of electric motors, and more particularly, to precise sensing and control of the position and orientation of a planar linear motor incorporating a monolithic planar alternating current (AC) magnetic sensor.
2. Description of Prior Art
Motors that can move in a straight line (linear motors) are well known in the art. Motors that can move freely in the plane are less well known, but several examples exist. For example, the planar linear motor (henceforth referred to simply as a planar motor) due to Sawyer (U.S. Pat. No. 3,376,578) can provide linear motion in two mutually orthogonal directions in the plane as well as a small rotation in the plane.
Such a planar motor generally combines four linear-motor sections into one forcer assembly that is capable of producing forces and torques in the plane. The forcer is magnetically attracted to a patterned iron platen surface while being forced away from the surface by an air bearing film; the equilibrium separation being typically 10 to 15 .mu.m. The motor sections have fine teeth [typically 0.5 mm (0.020 in.) wide on a 1.0 mm (0.040 in.) pitch] and the platen has a two-dimensional array of square teeth of corresponding width and pitch. After chemical or physical machining, the platen surface is planarized using epoxy to form the air-bearing surface. The combined motor sections making up the forcer ride above (or hang below) the platen (stator) surface, and typically operate on a flux-steering principle in open-loop microstepping mode. That is, a string of pulses from the control computer serves to increment counters which set proportional currents in the drive coils which, in turn, move the stable magnetic equilibrium point which, in turn, provides a force which moves the motor forward. These developments are chiefly due to Sawyer, and date from the late 1960s.
Planar motors have many desirable attributes. Commercial systems such as RobotWorld (V. Scheinman, "RobotWorld: a multiple robot vision guided assembly system," in Robotics Research, the Fourth International Symposium, Santa Cruz Calif., 1987, pp. 23-27, and "RobotWorld--unrolled motors turn assembly on its head," Industrial Robot, Vol. 20, No. 1, 1993, pp. 28-31) use forcers carrying vertical and rotational axes and vision cameras suspended from a platen ceiling for automated assembly. Similar systems have been developed by AT&T (P. F. Lilienthal, et al., "A flexible manufacturing workstation," AT&T Technical Journal, 1998, pp. 5-14) and Megamation (Anon., "Speed and precision from novel assembly robot," Assembly Automation, Vol. 9, No. 2, 1989, pp. 85-87) for a wide variety of automation applications such as the placement of surface-mount components on circuit boards (B. D. Hoffman, "The use of 2-D linear motors in surface mount technology," Proc. 5th Int'l SAMPE Electronics Conference, 1991, pp. 141-151).
While offering many benefits, current planar motion systems are severely limited because of their open-loop stepping operation which prevents the achievement of maximum potential performance. To help ensure against loss of synchrony (missing steps), only two-thirds to three-fourths of the available force margin is used, reducing the forcer's potential maximum acceleration and velocity. Even so, the forcer motors remain susceptible to loss of synchrony if large enough unanticipated external forces are acting. Additionally, settling times after moves are longer than desirable and there is no way to reject low-frequency external disturbances. The forcer has only moderate stiffness requiring high power dissipation when holding a position.
Many have recognized that these problems can be solved or considerably reduced in severity by incorporating a suitable position sensor that can accurately measure the relative displacements of forcer and platen at high enough bandwidth to be used for servo control for greatly improved performance. Among the possible sensing strategies are laser interferometry, tracking from light sources attached to the forcer, optical sensing of teeth in the platen, capacitive sensing of teeth, and magnetic sensing of teeth.
Interferometric or other optical tracking techniques are expensive and run into trouble when multiple forcers are used in a cluttered environment. On the other hand, sensors which are self contained and can be mounted on or incorporated into the forcer would appear to be the most desirable. Such sensors could use either magnetic, capacitive or optical principles to generate electrical output when the forcer is driven over the platen array. The output signals, either pulses or continuous waveforms, would correspond to the platen array dimensions. These could be used for closed-loop coarse distance control by pulse counting and/or intra-tooth fine control by interpolating the analog waveform.
Sawyer himself recognized the desirability of a platen tooth sensor and patented a method based on magnetic induction (U.S. Pat. No. 3,735,231). One embodiment of the sensor in U.S. Pat. No. 3,735,231 includes a four pole magnetic member having a pair of sense windings which can be in the form of a printed circuit board disposed on non-adjacent poles at the exposed end of the poles. The pair of windings provide outputs which are a periodic function of the head relative to the platen along a single axis. The patent does not teach control of the motor from the sensor signals.
U.S. Pat. No. 3,857,078 to Sawyer discloses a closed loop planar motor using magnetic sensing. For detection along each axis, two pickoff assemblies are utilized. Each pickoff includes two magnetic cores joined by a magnetic cross piece having a drive coil wrapped around it. Each magnetic core has two poles, each with three teeth. The two poles of one core are spaced in a phase quadrature relationship with the two poles of the other core. The flux in each core varies with the linear positioning of the pickoff relative to the platen and the fluxes in the two cores are in a quadrature relationship with each other. A sense coil is wound around an upper horizontal portion of each core member. The two sense coils provide quadrature related output signals having periodic relationships in accordance with the actual displacement of the head along the platen. This sensor, however, suffers from the disadvantage that since the magnetic path is not symmetrical on both sides of the drive coil the outputs have a large common mode (bias field) component which is not cancelled.
A magnetic sensing technique is disclosed by Brennemann, et al., ("Magnetic sensor for 2-D linear stepper motor," IBM Technical Disclosure Bulletin, Vol. 35, No. 1B, June 1992). This sensor is an AC magnetic sensor based on self inductance of coils integrated with a planar motor. The sensor includes four linearly shaped poles, each having a plurality of teeth. Two poles on the left are separated from the two poles on the right by a magnetic spacer. A sense coil (L1-L4) is wound around each of the poles. The sensor for each axis consists of eight coils wound on eight poles. Four poles are positioned in one quadrant of the forcer and four are positioned in the diagonally opposite quadrant. The inductance of a first sense coil L1 is at a maximum when the inductance of a second coil L2 is at a minimum and vice-versa. The sense coils L3 and L4 are in phase quadrature with the coils L1 and L2. Each four-pole sensor produces quadrature related output voltages which vary sinusoidally with forcer displacement along a single axis of the platen. The sensors can be used to measure displacement along one of two axes and rotation about the z axis. The above sensors are, however, relatively complex to make and use, expensive to manufacture, difficult to shield from unwanted external fields and have a relatively small signal. There is no discussion of motor control based on such signals.
U.S. Pat. No. 5,434,504 to Hollis, et al. discloses a position sensor for planar motors using inductive coupling to the platen teeth through planar drive and planar sense coils. In one embodiment, the sensor includes first and second magnetic members having teeth disposed relative to the teeth on the platen. A single turn planar drive winding is disposed around at least one of the teeth of the first and second magnetic members for producing a first and a second drive flux within each of the magnetic members. A single turn planar sense winding is disposed around at least one of the teeth of the first and second magnetic members for generating first and second outputs which are a periodic function of the position of the sensor relative to the platen. In another embodiment, the sensor includes two magnetic members each having four pole pieces. A drive winding is disposed on each member for establishing first and second fluxes in each member which are symmetrical about a center of each member. A sense winding is wound around the center poles of each member for measuring the relative flux therein and producing an output which is a periodic function of the position relative to the platen. A disadvantage is the lack of an ability to measure rotation in the plane with a single sensor substrate. Another disadvantage is the difficulty during manufacturing of aligning multiple sensor substrates in a single forcer. The patent does not incorporate a control element for control of the planar motor from the derived signals.
A position sensing technique based on sensing capacitance between patterned electrodes and the platen teeth is described in U.S. Pat. No. 4,893,071 to Miller. In one embodiment, chevron-shaped electrodes couple electrostatically to the platen teeth, from which position signals for control can be derived. A disadvantage of this approach is capacitance changes due to changing humidity and contamination between the electrodes and the platen surface.
An optical sensing technique using colored stripes similar to that used by an optical mouse device was patented by Hoffman and Pollack (U.S. Pat. No. 4,823,062). In this technique, optical filters are used to differentiate between stripes in orthogonal directions. A disadvantage of this approach is the need to interpose a pattern of colored stripes in the very thin air bearing that exists between the platen and the forcer, leading to reduced magnetic fields (or reduced air gap, requiring tighter manufacturing tolerances). Additionally, there are manufacturing challenges in producing large areas of precision made stripes and their subsequent bonding to the platen surface.
Another optical technique based on reflected light was developed by Nicolson, et al., ("Optical sensing for closed-loop control of linear stepper motors," Proc. Int'l Conf. on Advanced Mechatronics, Tokyo, Japan, August 1993). This technique uses light produced by light emitting diodes (LEDs) shining through slit-shaped masks and viewed by photodiode detectors to produce quadrature position signals from the platen teeth. A disadvantage is noisiness of the signals derived from the reflected light due to random scratches, dirt, and corrosion on the top surfaces of the platen teeth.
U.S. Pat. No. 5,324,934 to Clark discloses the use of fiber optics to determine the position, velocity, and direction of movement of a planar motor. There are a pair of channels, each one of which has two bundles of optical fiber. A first end of the optical fiber of each of the bundles is disposed within a narrow elongated slit. One of the bundles conveys light directed upon the opposite end of the bundles to the platen surface adjacent to the slit. The remaining bundles convey light reflected from the surface to a photodetector. The slits are spaced appropriately relative to the platen tooth pitch spacing so that both position and direction of motion can be ascertained. A disadvantage is complexity of manufacture and susceptibility to scratches and contamination.
U.S. Pat. No. 5,818,039 to Lampson describes an optical reflectance sensor which uses a charge-coupled device (CCD) detector to sense motion in the plane. A plurality of detectors is mounted onto a forcer to sense motion along a particular direction, with the detector being insensitive to motion along an orthogonal direction. Disadvantages include the complexity of optically coupling the CCD detectors through optical beam splitters and cylindrical lenses with its attendant bulkiness and cost. As with other optical methods, there is susceptibility to corrosion, dirt, and contamination.
Another optical technique was proposed by Brennemann, et al., ("An optical means of sensing position of a Sawyer motor on a magnetic grid surface," IBM Technical Disclosure Bulletin, vol. 37, pp. 375-378, May, 1994). This method is an improvement on conventional optical sensing of platen teeth. It uses a fluorescent dye embedded in the epoxy backfill between the platen teeth. When the dye is illuminated through slits with light of wavelength .lambda..sub.1, it re-radiates light at a wavelength .lambda..sub.2 &gt;.lambda..sub.1. By using optical filters to remove the reflected component .lambda..sub.1, only the component .lambda..sub.2 is detected, eliminating variations in reflectance due to scratches and contamination. Brennemann and Hollis published a paper ("Magnetic and optical-fluorescence position sensing for planar linear motors," Int'l Conf. on Intelligent Robots and Systems, Vol. III, August, 1995, pp. 101-107) making detailed comparisons between optical and magnetic methods.
All of the sensors and control methods for planar motors heretofore described suffer from a number of disadvantages:
(a) Sensors based on optical reflectance techniques are susceptible to scratches, dirt, and corrosion on the top surface of the platen. These sensors measure the perceived optical position of the platen teeth which may or may not represent the true position of the teeth. As a result, position signals for use by a control system are corrupted by spatial noise, i.e. as the planar motor carrying the sensing head moves across the regular teeth of the platen surface, the perceived position signals will be irregular to the extent that contamination effecting the optical signal is present.
(b) Sensors based on optical fluorescence techniques are bulky and difficult to integrate within a planar motor structure since the necessary optical components must occupy a three-dimensional extent. Additionally, these sensors cannot be used on conventional platen surfaces which lack a fluorescent dye. Incorporation of dye into the epoxy backfill of a platen surface requires an additional manufacturing step. Also, it is difficult to provide a uniform concentration of dye, and failing to do so introduces unwanted spatial noise in the position signals.
(c) Sensors based on capacitance techniques require incorporating shielded electrode arrays into the planar motor structure. These electrode arrays must be constructed in ways to eliminate common mode signals resulting from changes of flying height of the planar motor. The capacitance signal is subject to changes in humidity in the environment and to contamination, e.g dielectric films on the platen surface. These considerations make it difficult to provide reliable sensing and control.
(d) Sensors based on magnetic techniques using large three-dimensional core structures have the advantage that they are measuring the planar motor position with respect to intrinsically magnetic objects, namely the ferromagnetic teeth of the platen. On the other hand, most of the magnetic sensors described in the prior art use structures that are bulky, difficult to manufacture, and difficult to integrate with a planar motor. These sensors are also difficult to shield from unwanted magnetic interference, stemming chiefly from electric currents in adjacent motor windings. Signals from these sensors have not proved suitable for precise control of position and orientation of planar motors.
(e) Sensors based on magnetic techniques using small planar core structures have the advantage as in (d) that they are measuring the planar motor position with respect to intrinsically magnetic objects, namely the ferromagnetic teeth of the platen. On the other hand, they are difficult to integrate with a planar motor and require difficult precise alignment during manufacture.
What is needed is a platen sensor which is not susceptible to scratches, dirt, or other film contamination and which can readily be integrated with existing planar motor technology to provide precise lateral position as well as rotational orientation on the platen surface coupled with effective means of control so as to affect precise closed-loop planar motion.